CD38 is a 45 kD type II transmembrane glycoprotein with a long C-terminal extracellular domain and a short N-terminal cytoplasmic domain. The CD38 protein is a bifunctional ectoenzyme that can catalyze the conversion of NAD+ into cyclic ADP-ribose (cADPR) and also hydrolyze cADPR into ADP-ribose. During ontogeny, CD38 appears on CD34+ committed stem cells and lineage-committed progenitors of lymphoid, erythroid and myeloid cells. CD38 expression persists mostly in the lymphoid lineage with varying expression levels at different stages of T and B cell development.
CD38 is upregulated in many hematopoeitic malignancies and in cell lines derived from various hematopoietic malignancies, including non-Hodgkin's lymphoma (NHL), Burkitt's lymphoma (BL), multiple myeloma (MM), B chronic lymphocytic leukemia (B-CLL), B and T acute lymphocytic leukemia (ALL), T cell lymphoma (TCL), acute myeloid leukemia (AML), hairy cell leukemia (HCL), Hodgkin's Lymphoma (HL), and chronic myeloid leukemia (CML). On the other hand, most primitive pluripotent stem cells of the hematopoietic system are CD38−. CD38 expression in hematopoietic malignancies and its correlation with disease progression makes CD38 an attractive target for anti-CD38 antibody therapy.
CD38 has been reported to be involved in Ca2+ mobilization (Morra et al., 1998, FASEB J., 12: 581-592; Zilber et al., 2000, Proc. Natl. Acad. Sci. USA, 97: 2840-2845) and in the signal transduction through tyrosine phosphorylation of numerous signaling molecules, including phospholipase C-γ, ZAP-70, syk, and c-cbl, in lymphoid and myeloid cells or cell lines (Funaro et al., 1993, Eur. J. Immunol., 23: 2407-2411; Morra et al., 1998, FASEB J., 12: 581-592; Funaro et al., 1990, J Immunol, 145: 2390-2396: Zubiaur et al., 1997, J Immunol, 159: 193-205; Deaglio et al., 2003, Blood 102: 2146-2155; Todisco et al., 2000, Blood, 95: 535-542; Konopleva et al., 1998, J. Immunol., 161: 4702-4708; Zilber et al., 2000, Proc. Natl. Acad. Sci. USA, 97: 2840-2845; Kitanaka et al., 1997, J. Immunol., 159: 184-192; Kitanaka et al., 1999, J. Immunol., 162: 1952-1958; Mallone et al., 2001, Int. Immunol., 13: 397-409). On the basis of these observations, CD38 was proposed to be an important signaling molecule in the maturation and activation of lymphoid and myeloid cells during their normal development.
The exact role of CD38 in signal transduction and hematopoiesis is still not clear, especially since most of these signal transduction studies have used cell lines ectopically overexpressing CD38 and anti-CD38 monoclonal antibodies, which are non-physiological ligands. Because the CD38 protein has an enzymatic activity that produces cADPR, a molecule that can induce Ca2+ mobilization (Lee et al., 1989, J. Biol. Chem., 264:1608-1615; Lee and Aarhus, 1991, Cell Regul., 2: 203-209), it has been proposed that CD38 ligation by monoclonal antibodies triggers Ca2+ mobilization and signal transduction in lymphocytes by increasing production of cADPR (Lee et al., 1997, Adv. Exp. Med. Biol., 419: 411-419). Contrary to this hypothesis, the truncation and point-mutation analysis of CD38 protein showed that neither its cytoplasmic tail nor its enzymatic activity is necessary for the signaling mediated by anti-CD38 antibodies (Kitanaka et al., 1999, J. Immunol., 162: 1952-1958: Lund et al., 1999, J. Immunol., 162: 2693-2702; Hoshino et al., 1997, J. Immunol., 158, 741-747).
Evidence for the function of CD38 comes from CD38−/− knockout mice, which have a defect in their innate immunity and a reduced T-cell dependent humoral response due to a defect in dendritic cell migration (Partida-Sanchez et al., 2004, Immunity, 20: 279-291; Partida-Sanchez et al., 2001, Nat. Med., 7: 1209-1216). Nevertheless, it is not clear if the CD38 function in mice is identical to that in humans since the CD38 expression pattern during hematopoiesis differs greatly between human and mouse: a) unlike immature progenitor stem cells in humans, similar progenitor stem cells in mice express a high level of CD38 (Randall et al., 1996, Blood, 87: 4057-4067; Dagher et al., 1998, Biol. Blood Marrow Transplant, 4: 69-74), b) while during the human B cell development, high levels of CD38 expression are found in germinal center B cells and plasma cells (Uckun, 1990, Blood, 76: 1908-1923; Kumagai et al., 1995, J. Exp. Med., 181: 1101-1110), in the mouse, the CD38 expression levels in the corresponding cells are low (Oliver et al., 1997, J. Immunol., 158: 1108-1115; Ridderstad and Tarlinton 1998, J. Immunol., 160: 4688-4695).
Several anti-human CD38 antibodies with different proliferative properties on various tumor cells and cell lines have been described in the literature. For example, a chimeric OKT10 antibody with mouse Fab and human IgG1 Fc mediates antibody-dependent cell-mediated cytotoxicity (ADCC) very efficiently against lymphoma cells in the presence of peripheral blood mononuclear effector cells from either MM patients or normal individuals (Stevenson et al., 1991, Blood, 77: 1071-1079). A CDR-grafted humanized version of the anti-CD38 antibody AT13/5 has been shown to have potent ADCC activity against CD38-positive cell lines. Human monoclonal anti-CD38 antibodies have been shown to mediate the in vitro killing of CD38-positive cell lines by ADCC and/or complement-dependent cytotoxicity (CDC), and to delay the tumor growth in SCID mice bearing MM cell line RPMI-8226 (WO2005/103083 A2). On the other hand, several anti-CD38 antibodies, IB4, SUN-4B7, and OKT10, but not IB6, AT1, or AT2, induced the proliferation of peripheral blood mononuclear cells (PBMC) from normal individuals (Ausiello et al. 2000, Tissue Antigens, 56: 539-547).
Some of the antibodies of the prior art have been shown to be able to trigger apoptosis in CD38+ B cells. However, they can only do so in the presence of stroma cells or stroma-derived cytokines. An agonistic anti-CD38 antibody (IB4) has been reported to prevent apoptosis of human germinal center (GC) B cells (Zupo et al. 1994, Eur. J. Immunol., 24:1218-1222), and to induce proliferation of KG-1 and HL-60 AML cells (Konopleva et al. 1998, J. Immunol., 161: 4702-4708), but induces apoptosis in Jurkat T lymphoblastic cells (Morra et al. 1998, FASEB J., 12: 581-592). Another anti-CD38 antibody T16 induced apoptosis of immature lymphoid cells and leukemic lymphoblast cells from an ALL patient (Kumagai et al. 1995, J. Exp. Med., 181: 1101-1110), and of leukemic myeloblast cells from AML patients (Todisco et al. 2000, Blood, 95: 535-542), but T16 induced apoptosis only in the presence of stroma cells or stroma-derived cytokines (IL-7, IL-3, stem cell factor).